1. Field of the Invention
The present invention relates to a hoop for a CVT (continuously variable transmission) belt for an automobile, and more particularly, relates to a technique for enhancing the fatigue strength by minimizing the effects of foreign matter.
2. Description of the Related Art
A CVT belt is composed of plural push blocks linked annularly by a metal hoop. The hoop is exposed to repeated bending loads, and high fatigue strength is therefore required. As a technique for enhancing the fatigue strength of the hoop, various methods have been proposed. For example, (1) Japanese Patent Application Laid-open (JP-A) No. 11-293407 discloses maraging steel in which particle sizes of Ti type inclusions are restricted to 8 μm or less as a hoop material, and (2) JP-A No. 2001-64755 discloses maraging steel in which particle sizes of nonmetallic inclusions are restricted to 30 μm or less. Aside from such improvements in materials, improvements to the hoop itself have also been proposed for example, (3) JP-A No. 62-80322 discloses a technique for removing edges from hoop margins by barrel polishing the hoop, and (4) JP-A No. 1-142022 discloses a technique for enhancing the fatigue strength by gas nitriding treatment of the hoop. Furthermore, (5) JP-A No. 63-96258 discloses a technique for enhancing the fatigue strength by shot peening on the hoop.
To enhance the fatigue strength of the hoop remarkably, it may be considered to combine the means for improving the material and the means for improving the hoop itself in the conventional arts. However, expected effects are not obtained in practice. For example, when the hoop is made of the material disclosed in (1) JP-A No. 11-293407, and it is treated by shot peening disclosed in (5) JP-A No. 63-96258, or by barrel polishing disclosed in (3) JP-A No. 62-80322 to remove edges instead of (or in addition to) shot peening, the fatigue strength is not enhanced remarkably. The reason is that shot or the like is driven into or dents the hoop surface by shot peening. Therefore, even if materials with small inclusions as disclosed in (1) JP-A No. 11-293407 or (2) JP-A No. 2001-64755 are used, foreign matter infiltrates into the surface in the process of manufacturing a hoop product, and such foreign matter may be an initiation of fatigue rupture, thereby lowering the fatigue strength.
As a means for avoiding such phenomena, it is generally known to remove exogenous foreign matter by electrolytic polishing to remove the surface layer of the hoop after barrel polishing or shot peening. By such means, however, the time and labor for manufacture are increased, and the fatigue strength is reduced if the portion provided with residual compressive stress by shot peening is removed.